List of communities:

https://code.djangoproject.com/wiki/DjangoResources#Communit...

Mailing lists:

https://www.djangoproject.com/community/ on the right

Sizes of communities:

reddit:

http://www.reddit.com/r/django - 4,181 readers

http://www.reddit.com/r/rails - 1,994 readers

http://www.reddit.com/r/rubyonrails - 921 readers

Stackoverflow:

http://stackoverflow.com/questions/tagged/django 20k tagged

http://stackoverflow.com/questions/tagged/ruby-on-rails 40k tagged